Clonal subpopulations have been isolated from lymphosarcoma P1798, and glucocorticoid-resistant variants were selected in culture and in vivo. Variants that are resistant to the cytolytic effects of glucocorticoids in vivo express near wild-type levels of glucocorticoid binding and normal levels of nuclear translocation of the hormone-receptor complex. Injection of dexamethasone (dex) into tumor-bearing mice results in essentially complete saturation of the receptor within 2 hrs, and wild-type and resistant tumor cells are identical with respect to the kinetics and extent of hormone binding in vivo. The modal number of chromosomes does not change during selection in vivo. P1798 cells were infected with mouse mammary tumor virus (MMTV), and induction of MMTV RNA synthesis was measured as an indicator of glucocorticoid receptor function. Treatment with dex in culture and in vivo stimulated MMTV expression in wild-type cells and in resistant variants. Moreover, variants that were selected for resistance in vivo were completely sensitive to the receptor-mediated antiproliferative effects of dex in culture. These data clearly indicate that selection for glucocorticoid resistance in vivo does not result in loss of receptor function. Glucocorticoid-resistant variants selected in culture expressed classical receptor-defective phenotypes including reduced total hormone binding and nuclear translocation of the hormone-receptor complex. Such variants grew at wild-type rates in the presence of dex in culture, but all such isolates were sensitive to the cytolytic effects of glucocorticoids in vivo. In summary, variants selected in vivo expressed functional glucocorticoid receptors and were sensitive to glucocorticoids in culture. Conversely, variants selected in culture expressed defective receptors and were sensitive to the cytolytic effects of the hormone in vivo. We conclude that the pharmacological effects of glucocorticoids in vivo are independent of, or superimposed upon, the physiological (i.e., receptor-mediated) effects of the hormone. Our working hypothesis is that glucocorticoid receptors may have little or nothing to do with cytolysis in vivo. (D)